The present invention relates to an air intake system for an internal combustion engine and more particularly relates to an air intake system adapted for use in generating an adequate swirling motion of intake air introduced into combustion chambers of a direct injection-type diesel engine.
It is known that in a direct injection-type diesel engine, generating a swirling motion of intake air about the cylinder axis when the intake air is introduced into the combustion chamber of the cylinder is extremely effective for obtaining a well blended mixture of the intake air and fuel within the combustion chamber and thereby causing complete combustion of the mixture.
It is also known that the intensity of the swirling motion of the intake air must be regulated so as to be suited for a given shape of the combustion chamber and for a specific type of fuel injection system employed in the diesel engine. Further, the intensity regulation of the swirling motion must be performed without decrease in weight flow of the intake air flowing into the combustion chamber, so that increases in output power and combustion efficiency of the diesel engine is achieved. Thus, there have already been proposed diverse types of methods for regulating the intensity of the swirling motion during suction of the intake air into a diesel engine. However, these diverse methods can be classified by the following two types. That is, in the first method, an air intake passageway is sloped so that the intake air is admitted into a cylinder in the tangential direction of the cylinder bore. In this sloping air intake passageway, a desired swirling motion of the intake air is produced by selecting the sloping angle in relation to the axis of the cylinder and by selecting the angle of the passageway in relation to the tangential direction of the cylinder bore. Thus, it can be appreciated that in the above first type method, the regulation of the intensity of the swirling mothin is considerably easy. However, there is such a disadvantage that when production of a strong swirling motion is required, it is impossible to prevent a decrease in the amount of intake air flowing into the combustion chamber. Further, uniformity in production of the sloping air intake passageway is so sensitive, that the slightest change affects the intensity of the swirling motion. Consequently, in the case of a multi-cylinder engine, such change in production uniformity often makes it difficult to acquire optimum combustion of the air fuel mixture in each cylinder of the multi-cylinder engine.
In the second type method, the air intake passage is formed into a spiral shape so that the swirling motion of the intake air is induced while passing along the spiral-shaped air intake passageway. In the spiral-shaped air intake passageway, a stable swirling motion can surely be obtained. However, since the swirling motion of the intake air is compulsorily produced by the spiral-shaped passageway, the swirling motion becomes relatively strong and further, a control for attenuating the strong swirling-motion to an appropriate level is very difficult. Furthermore, if the intensity of the swirling motion of intake air is set to be appropriate at a medium engine speed, in the range of engine speed other than medium engine speed, the intensity of the swirling motion can not be appropriate. That is, at a high engine speed range, the swirling motion is very strong, while at a low engine speed range, the swirling motion is very weak. Consequently, in these two engine speed ranges, diffusion of the atomized fuel within the combustion chamber and mixing of the atomized fuel with the intake air are not suitable for causing good combustion within the combustion chamber.
In order to eliminate the disadvantages of the above-mentioned two types of methods, a further conventional method comprising a combination of the sloping intake air passageway and the spiral-shaped intake air passageway has been developed. In this conventional method, the intake valve is devised so that an arcuate-shaped masking plate is provided on one side of the intake valve so as to circumferentially extend along the margin of this one side. Thus, the intake air is admitted into the combustion chamber only around a part of the periphery of the intake valve and in the desired direction causing the swirling motion of the intake air. In this case, the intensity of the swirling motion of the intake air can be regulated by adjusting the position of the masking plate with respect to the air intake passageway during diesel engine assembly. However, provision of the arcuate masking plate on the one side of the intake valve results in the fact that effective passage of the intake air around the intake valve is restricted thereby causing a disadvantageously large decrease in the amount of intake air. Therefore, this latter conventional method could not be a complete method from the point of view of acquiring an increase in diesel engine power, enhancement of the combustion efficiency of the diesel engine and a reduction in harmful exhaust emissions from the diesel engine.